The present invention relates to an electroviscous fluid, the viscosity of which can be controlled by applying voltage, and in particular to an electroviscous fluid, in which viscosity is changed rapidly and reversibly from low temperature range to high temperature range when voltage is applied and long and stable electroviscous effect is obtained and which can be used for electrical control of mechanical devices such as clutch, valve, shock absorber, etc.
Electro-rheological fluid or electroviscous fluid, in which viscosity of the fluid is changeable by the application of voltage, has been known since many years ago (Duff, A. W., Physical Review, Vol. 4, No. 1 (1896) 23). Early studies on electroviscous fluid were concentrated on the system containing liquid only, and the effect was not strong enough. Later, studies have been made on the electroviscous fluid of solid disperse system, where considerable electroviscous effect could be obtained.
For example, Winslow proposed an electroviscous fluid, using paraffin, silica gel powder and adding water to make the system slightly electroconductive (Winslow, W. M., J. of Applied Physics, Vol. 20 (1949) 1137). From this study by Winslow, the electroviscous (electro-rheological) effect of the electroviscous fluid is called ER effect or Winslow effect.
On the other hand, study has also been made on the mechanism of electroviscous effect (ER effect) in the electroviscous fluid. For example, Klass reported that each particle, i.e. the dispersed phase in electroviscous fluid, generates the induced polarization of the double layer in electrical field and this was the primary cause of such effect (Klass, D. L., et al., J. of Applied Physics, Vol. 38, No. 1 (1967) 67). If this is explained from the principle of the electric double layer, the ions adsorbed on the dispersed solids (such as silica gel) are evenly arranged on outer surface of dispersed solids when E (electric field)=0, while polarization occurs in the ion distribution when E (electric field)=finite value and each particle exerts electrostatic action on each other in electric field. Thus, each particle forms bridge (cross-linkage) between electrodes and the shear-resistant force to the external stress, i.e., ER effect.
With full consideration given on the mechanism of ER effect in the electroviscous fluid of this solid dispersing system, various proposals have been made to increase and to stabilize the viscosity of electroviscous fluid. For example, a proposal was made to use silicon dioxide type fine particles, on which ferroelectric powder and small quantity of water are adsorbed (Japanese Provisional Patent Publication 53-17585), whereas this is disadvantageous and unsatisfactory in various points such as the response, the reproducibility of ER effect, low electroviscous effect, the stability of ER effect for a long time, etc. Particularly, the conventional electroviscous fluid containing moisture is disadvantageous in that water is evaporated at high temperature of more than 80.degree. C. Further, even at less than 80.degree. C., there are also the problems such as the unstability by the migration of moisture on the surface of the particles, the problem of durability by elution of electrode metal (such as copper) when high electric field is applied, the enhancement of ionization by temperature increase and the increase of electric current, or unstable temperature property, and other problems caused by the presence of moisture.
It is an object of this invention to offer an electroviscous fluid, by which it is possible to maintain ER effect stably up to high temperature of 100.degree. C. and to achive high durability.